1. Field of the Invention
The present invention relates to an amplifier for amplifying signals in the microwave and milliwave frequency bands, and relates particularly to an amplifier having a function for compensating variations in amplifier gain resulting from changes in amplifier temperature.
2. Description of the Prior Art
The gain of microwave and milliwave amplifiers and amplifier modules comprising such amplifiers tends to vary with the environmental temperature in which the amplifier or amplifier module is used. More particularly, the gain of such an amplifier or amplifier module tends to rise when the environmental temperature drops, and to drop when the environmental temperature rises. The range of this change is particularly pronounced in multiple stage amplifiers, and in cases can interfere with stable operation of the overall system.
For example, in a power amplifier module comprising three two-stage amplifier MMIC devices with different outputs in series, a 100.degree. C. change in the environmental temperature results in a 12 dB change in gain if the temperature-dependent gain change per amplifier stage is 2 dB/10.degree. C. The distortion characteristic of such a power amplifier module thus deteriorates when the environmental temperature is low and gain is greatest. Loss of power and efficiency are also potential problems when the environmental temperature is high and gain is low.
To compensate for such changes in gain resulting from temperature variations, a temperature compensation circuit can be connected in series with the gate terminal of an amplification FET for directly controlling an applied bias voltage. This temperature compensation circuit compensates for a drop in the operating current of the amplifier resulting from a rise in temperature by supplying an appropriate current to the gate. The internal operating current of the amplifier is thus held to a constant level within a specified operating temperature range.
A drawback to the above-noted conventional temperature compensation circuit is that a separate external power supply is needed to supply the required bias voltage to the gain control terminal, that is, the gate of the amplification FET. It is also necessary to measure the temperature characteristics of amplifier gain to predetermine the bias required at a particular temperature, and provide in the amplifier system or module a further mechanism for actually applying this information during system or module operation. This complicates circuit design and configuration, and thus leads to increased cost.
A power amplification apparatus capable of temperature compensation without having a temperature compensation circuit as described above is described in Japanese Patent Laid-Open Publication No. 9-139630. This power amplification apparatus is formed on a GaAs substrate, and comprises an amplification FET for amplifying a high frequency power supply, and a self-biased gate bias circuit in series with a resistor and a temperature compensation FET formed on a GaAs substrate. One terminal of the gate bias circuit is connected to a power supply, and the other to ground. To exhibit opposite temperature characteristics, the gate orientation of the amplification FET is set at 90.degree., and the gate orientation of the temperature compensation FET is set at 0.degree., relative to the orientation flat of the GaAs substrate.
As the environmental temperature rises and the operating current of the amplification FET drops, the operating current of the temperature compensation FET rises. On the other hand, when the environmental temperature drops and the operating current of the amplification FET rises, the operating current of the temperature compensation FET drops.
With the power amplification apparatus described above, a temperature compensation FET with the above-noted temperature characteristic is used as a resistor, and is connected in series with a normal resistor to form a gate bias series circuit. Using resistance division, this gate bias series circuit works to stabilize the power amplification apparatus output by suppressing changes in the amplification factor in response to temperature changes.
Process control can be expected to become difficult, however, when FETs with significantly different characteristics are formed on a single wafer to achieve a power amplification apparatus as described above, with particular performance characteristics.